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A team led by Lee Burger has announced the discovery of a new species of early human.

Professor Berger, an American palaeoanthropologist working at the University of the Witwatersrand, Johannesburg, is known for Australopithecus sediba, announced in 2010 and at the time was the first new hominin species to be discovered in South Africa for decades. The discovery was when Matthew, Lee’s nine-year-old son, discovered a hominin collar bone embedded in a rock at Malapa, part of a now-eroded cave system near to Sterkfontein and Swartkrans, where many important hominin finds have been made.

Berger felt other cave systems in South Africa had the potential to yield hominin fossils, so in 2013 he recruited a team of cavers to search the Rising Star cave system, 50 km (30 miles) northwest of Johannesburg. The cave has been well explored over the years, but the team came across a narrow 18 cm (7 in.)-wide shaft that dropped vertically for 12 m (39 ft.) into an unexplored chamber. The cavers descended into the chamber and saw a fossil skull and jawbone lying on the floor of the cave. Berger believed that there were hominin fossils and obtained funding from the National Geographic for an expedition.

But to access what became known as Dinaledi Chamber (‘chamber of stars’) was problematic. Before they could even reach the narrow shaft leading down to the chamber, researchers, researchers would have to pass through another tiny shaft known as Superman’s Crawl and then climb a steep section known as Dragon’s Back. Berger placed an advertisement on Facebook for ‘small, skinny’ scientists believing at best there might be three or four people in the world who would fit the criteria. In the event, within days 57 suitable candidates had applied from which he chose six, all women. Within a month, the Rising Star Expedition had set up camp at the cave system and excavations commenced. Working in six-hour shifts, the six women soon recovered more fossil material than had been found in the whole of South Africa in the previous 90 years. Meanwhile, back on the surface, a large team began preparing and cataloguing the fossils, making full use of social media to report progress. A total of 1,550 fossils were recovered, comprising 15 individuals, including males, females and infants.

Berger then invited thirty young postdoctoral researchers from fifteen countries to help him evaluate the haul at a workshop in Johannesburg. They were accompanied by twenty of Berger’s more senior colleagues, who had worked with him on the Australopithecus sediba discovery. This unusual move did not please everybody and some questioned the wisdom of handing over such important fossils to inexperienced researchers.

The findings have now been announced. The remains represent a new human species, Homo naledi, named for the word ‘star’ in the local Sotho language. The new species is comparable in height and weight to a small-bodied modern human or a large australopithecine, with an estimated stature of around 1.5 m (5 ft.) and weighing 40 to 55 kg (88 to 121 lb.). The brain is tiny, ranging from 465 to 560 cc, overlapping entirely with the range of values known for australopithecines. The reconstructed skeleton exhibits both humanlike and apelike features, but in a combination that has not been seen with other hominins. The feet and lower limbs are humanlike, but the upper thighbone, pelvis and shoulders are apelike. The hands and wrists are humanlike, though the fingers are curved suggesting that it spent some of its time in the trees as well as on the ground. Overall, Homo naledi is the most primitive, small-brained hominin ever to have been included in Homo, but the shape of cranium and lower jawbone and the dentition suggest that it is human rather than an australopithecine.

Unfortunately, no dates have yet been published for the fossils. They are presumably too old to be radiocarbon dated, but there is no readily-datable material in the chamber. Calcium carbonate flowstones have been found to have been contaminated with materials from associated muds, making them unsuitable for uranium series dating. All we currently have to go on is the primitive characteristics such as the small brains. These suggest that Homo naledi emerged close to the base of the human family tree 2.5 to 2.8 million years ago. But until we have dates for the fossils, or other fossils turn up that can be dated, it will be difficult to say just where Homo naledi fits into the overall picture of human evolution.

Also troublesome is the question of how the fossils reached Dinaledi Chamber in the first place. There is a near-absence of non-hominin fossils in Dinaledi Chamber – yet these are abundant in the adjacent Dragon’s Back. This rules out the remains having being swept into Dinaledi Chamber by a flash flood, as this would have left a mixture of hominin and non-hominin remains in both chambers. Carnivores are also ruled out: even if there was a carnivore that preyed exclusively on Homo naledi, why would it drag its prey into such an inaccessible location? In any case, none of the bones showed any evidence of having been gnawed by carnivores. Nor does it seem that the hominins fell down a shaft leading into the cave from the surface: there is no evidence that any such shaft had ever existed. The fossils accumulated over time, so it can also be ruled out that a single group entered the chamber for some reason and then become trapped there.

The only obvious explanation is that the remains were deliberately placed in the chamber as part of a post-mortem ritual, although there is no evidence for such rituals until much later. Mass deposition of corpses is first seen at the cave site of Sima de los Huesos in Spain, 430,000 years ago. Even this was nothing more than a hygienic disposal of the corpses rather than any form of ritual. Also, unlike Homo naledi, the brain size of the Sima people was only slightly below that of modern people. In any case, even hygienic disposal seems unlikely as there is no evidence that Rising Star was ever inhabited, and there would surely be no need to use such an inaccessible chamber.

Taken at face value, the evidence from Dinaledi Chamber suggests that early humans were far more behaviourally complex than has long been believed. However, it is probably too soon to jump to conclusions and all that can safely be said is that we don’t yet know how the fossils reached the cavern.

References:

Berger, L., Hawks, J., de Ruiter, D. & Churchill, S., 2015. Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. eLife.

Dirks, P. et al., 2015. Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa. eLife.